The epidermal growth factor receptor (EGFR) is the 170 kilodalton membrane glycoprotein product of the proto-oncogen c-erb B(1). The EGFR gene is the cellular homolog of the erb B oncogene originally identified in avian erythroblastosis viruses(1-2). Activation of this oncogene by gene amplification has been observed in a variety of human tumors(3-6).
EGFR has been demonstrated to be overexpression on many types of human solid tumors(7), including lung, colon, breast, gastric, brain, bladder, head and neck, ovarian, kidney and prostate carcinomas(7). One major difference between v-erb B oncogenes and the normal EGFR gene is that the viral oncogenes are amino-truncated versions of the normal receptor: they lack most of the extracytoplasmic domain but remain the transmembrane and tyrosine kinase domains(8-11). This results in a protein that is unable to bind epidermal growth factor (EGF) but can still phosphorylate other substrates(14-15).
A variety of genetic alterations can occur in viral erb B oncogenes, e.g. amino acid substitutions and deletions in the carboxy terminus of the gene. However, available evidence argues that the amino truncation is critical to carcinogenesis. Amino truncations are a feather of all v-erb B oncogenes, including those that arise by promoter insertion or retroviral transduction(13, 16). In contrast, carboxy-terminal deletions appear to be associated only with tumors that arise through retroviral transduction and seem to determine host range and tumor type specificity(11, 15). Transfection experiments with amino-truncated avian c-erb B genes or viral oncogene-human EGF receptors demonstrate that this deletion is sufficient alone to create cell transformation(16-17).
Amplification of the EGFR gene occurs in 40% of the malignant human gliomas. Rearrangement of the receptor gene is evident in many of the tumors with gene amplification(3-7). The structural alterations seem to preferentially affect the amino terminal half of the gene(6, 18).
There are eight major variants of EGFR that are known: 1) EGFRvI lacks a majority of the extracellular domain of EGFR. 2) EGFRvII consists of an 83 aa in-frame deletion in the extracellular domain of EGFR. 3) EGFRvIII consists of a 267 aa in-frame deletion in the extracellular domain of EGFR. 4) EGFRvIV contains deletions in the cytoplasmic domain of EGFR. 5) EGFRvV contains deletions in the cytoplasmic domain of EGFR. 6) EGFR.TDM/2-7 contains a duplication of exons 2-7 in the extracellular domain of EGFR. 7) EGFR.TDM/18-26 contains a duplication of exons 18-26 in the tyrosine kinase domain of EGFR. 8) In addition, there is a second, rarer, EGFRvIII mutant (EGFRvIII/Δ12-13) that possesses a second deletion that introduces a novel histidine at the junction of exons 11 and 14(24).
EGFRvIII is the most commonly occurring variant of the epidermal growth factor (EGF) receptor in human cancers(24). During the process of gene amplification, a 267 amino acid deletion occurs in the extracellular domain creating a novel junction (glycine). EGFRvIII is not known to be expressed on any normal tissues(19, 20). Yet, EGFRvIII shows significant expression in tumor cells, e.g., 27˜76% breast cancer biopsies express EGFRvIII(21), 50˜70% gliomas express EGFRvIII(19, 22), 16% NSCL cancers express EGFRvIII(23), and 75% ovarian cancers express EGFRvIII(22).
A method of treating cancers which over-express EGFRvIII involved the use of a tumor-specific ribozyme targeted specifically to the variant receptor which did not cleave normal EGFR. The ribozyme was found to significantly inhibit breast cancer growth in athymic nude mice(25).
In addition, the substitution of the deleted 267 amino acid with a Glycine creates a unique junction that may be capable of antibody targeting. Further, in view of EGFRvIII's expression in certain tumors and its lack of expression in normal tissues, EGFRvIII may be an ideal target for drug targeting in tumor therapy. In particular, EGFRvIII would appear to be an ideal candidate for immunoconjugate therapy of tumors. The monoclonal antibody against EGFRvIII (or the immunoconjugate of anti-tumor agent and toxic) can cause antibody-dependent cell-mediated cytotoxicity (ADCC) or kill the cells in vivo, leading to eliminating the tumor cells with EGFRvIII expression.
At present, a number of antibodies directed against EGFR have been obtained at home and abroad, but these antibodies still are not very ideal because of causes like their having no or less specificity for EGFRvIII.
Therefore, in the art it is urgent to produce a specific monoclonal antibody directed against EGFRvIII with highly specificity for this receptor but not the wtEGFR and with other excellent characteristics, leading to developing a new drug with distinct therapeutic effect.